An axial flow rotary machine, such as a gas turbine engine for an aircraft, includes a compression section, a combustion section and a turbine section. A flow path for hot working medium gases extends axially through the engine. The flow path for hot gases is generally annular in shape.
As working medium gases are flowed along the flow path, the gases are compressed in the compression section causing the temperature and pressure of the gases to rise. The hot, pressurized gases are burned with fuel in the combustion section to add energy to the gases. These gases are expanded through the turbine section to produce useful work and thrust.
The engine has a rotor assembly in the turbine section which is adapted by a rotor disk and blades extending outwardly therefrom to receive energy from the hot working medium gases. The rotor assembly extends to the compression section. The rotor assembly has compressor blades extending outwardly across the working medium flow path. The high-energy working medium gases in the turbine section are expanded through the turbine blades to drive the rotor assembly about its axis of rotation. The compressor blades rotate with the rotor assembly and drive the incoming working medium gases rearwardly, compressing the gases and imparting a swirl velocity to the gases.
Each rotor blade has an airfoil to direct the hot working medium gases through the stage of rotor blades and to receive work from the gases. As a result, the airfoils are bathed in hot working medium gases during operation causing thermal stresses in the airfoils. These thermal stresses affect the structural integrity and fatigue life of the airfoil. In addition, rotational forces acting on the rotor blade as the rotor blade is driven about the axis of rotation further increase the stresses to which the blade is subjected.
Rotor blades are typically cooled to reduce thermal stresses and thereby provide the rotor blade with a satisfactory structural integrity and fatigue life.
An example of such a rotor blade is shown in U.S. Pat. No. 4,474,532 entitled "Coolable Airfoil For a Rotary Machine", issued to Pazder and assigned to the assignee of this application. Another example of a coolable rotor blade is shown in U.S. Pat. No. 4,278,400 issued to Yamarik and Levengood entitled "Coolable Rotor Blade" and assigned to the assignee of this application. Each of these rotor blades is provided with a plurality of cooling air passages on the interior of the blade. Cooling air is flowed through the passages to the rearmost portion of the rotor blade, commonly referred to as the trailing edge, from whence the cooling air is exhausted into the working medium flow path.
As shown in Yamarik, the trailing edge region of the blade has a plurality of pedestals in the trailing edge region to increase heat transfer. More and more intricate constructions have been formed to increase heat transfer to the cooling air. These include a construction in which a pair of spanwisely extending ribs in the trailing edge region have a plurality of holes for directing cooling air on adjacent structure to improve heat transfer in the trailing edge region of the airfoil. The ribs are relatively small in the chordwise direction and relatively long in the spanwise direction.
During the casting process of such an airfoil, a ceramic core (which defines the openings and structure in the trailing edge region) is disposed on the interior of an airfoil shaped mold. Molten metal is poured around the core, rushing into the mold during the pouting process. The molten metal fills openings to form solid structure and flows around solid ceramic core material to form holes, such as the holes in the ribs. As the molten metal enters the structure, portions of the core in the trailing edge region may collapse resulting in an unusable casting.
The above art notwithstanding, scientists and engineers working under the direction of applicant's assignee are seeking to develop coolable airfoils made from cores which resist the forces imposed on the core during the casting process and which form a cooling structure in the finished airfoil having a satisfactory configuration.